Processes governing arsenic retardation on Pleistocene sediments: Adsorption experiments and model‐based analysis

Abstract

In many countries of south/south‐east Asia, reliance on Pleistocene aquifers for the supply of low‐arsenic groundwater has created the risk of inducing migration of high‐arsenic groundwater from adjacent Holocene aquifers. Adsorption of arsenic onto mineral surfaces of Pleistocene sediments is an effective attenuation mechanism. However, little is known about the sorption under anoxic conditions, in particular the behavior of arsenite. We report the results of anoxic batch experiments investigating arsenite (1–25 µmol/L) adsorption onto Pleistocene sediments under a range of field‐relevant conditions. The sorption of arsenite was nonlinear and decreased with increasing phosphate concentrations (3–60 µmol/L) while pH (range 6–8) had no effect on total arsenic sorption. To simulate the sorption experiments, we developed surface complexation models of varying complexity. The simulated concentrations of arsenite, arsenate, and phosphate were in good agreement for the isotherm and phosphate experiments while secondary geochemical processes affected the pH experiments. For the latter, the model‐based analysis suggests that the formation of solution complexes between organic buffers and Mn(II) ions promoted the oxidation of arsenite involving naturally occurring Mn‐oxides. Upscaling the batch experiment model to a reactive transport model for Pleistocene aquifers demonstrates strong arsenic retardation and could have useful implications in the management of arsenic‐free Pleistocene aquifers.

Biogeochemical phosphorus cycling in groundwater ecosystems – Insights from South and Southeast Asian floodplain and delta aquifers

“…A similar transport behavior was reported by Robertson (2008) and Harman et al (1996), who investigated the movement of PO4 3-plumes in sandy aquifers under anoxic conditions. They considered adsorption processes to be the only attenuation mechanism in operation, with the exhaustion of available adsorption sites resulting in PO4 3concentrations of up to 1.5 mg L -1 PO4-P. (calculated as the difference between the initial and resulting TP content after the 28 days of in situ exposure) yielded similar results (3.2 to 14.2 mg kg -1 ) compared to a laboratory batch sorption experiment previously conducted by Rathi et al (2017) at pH values of 6.1 to 7.9 using the same Pleistocene sediment material. Among the synthetic Fe(III)-(hydr)oxides, hematite-coated sand showed the highest TP contents after exposure (up to 13.1 mg kg -1 ), which is consistent with values reported by Colombo et al (1994) for laboratory batch sorption experiments at pH 6.…”

“…This retardation is primarily attributed to the presence of Fe(III)-(hydr)oxides in the Pleistocene aquifer sediments, which are characterized by a high adsorption affinity for PO4 3- (Borggaard 1983). Laboratory batch experiments confirmed this high PO4 3sorption capacity for Pleistocene sediment material from Van Phuc (Rathi et al 2017).…”

Arsenic mobilization in the piedmont area of the Hetao basin: an insight from a reactive transport model

“…Arsenic(V) and As(III) adsorption on representative sediments were studied, including As(V) or As(III) adsorption isotherm, pH effect, and competitive adsorption by HCO3and PO4 3- [8,13]. The data were then used with a non-electrostatic generalized composite surface complexation modeling (GC-SCM) [8,14]. The established GC-SCMs provided were input into the reactive transport model to simulate As sorption.…”

“…However, most of the surface complexation models (e.g. DDL, CD-MUSIC) incorporated in the reactive transport model have used synthesized Fe(III) oxides as the adsorbent [5,6], with only few attempts using sediments [7,8]. Therefore, building a reactive transport model coupling As adsorption onto sediments should be plausible.…”

Quantification of arsenic adsorption and oxidation on manganese oxides

“…and specific Mn mineralogy. So far, only a few attempts have been made to quantify As(III) oxidation by Mn oxides (e.g., Amirbahman et al, 2006;Rathi et al, 2017) and they do not follow a rigorous process-based modelling approach to interrogate the intermediate surface reactions and the prevailing geochemical conditions that influence the extent and rate of As(III) oxidation. This study aims to review the known As(III) oxidation mechanisms and to develop and evaluate suitable process-based modelling approaches against the experimental data available in the literature.…”